This invention relates to techniques for maintaining a minimum voltage value on a CMOS memory to maintain, in the event of a power outage, the data stored in the memory.
Repertory dialers are devices used in conjunction with telephone substations for storing a plurality of telephone numbers, each of which can be actuated, for example, by pressing a single pushbutton. Such apparatus must of course include some sort of a memory for storing the telephone number data, and appropriate apparatus for giving proper access. The memory may comprise a magnetic tape, but experience has shown that the mechanical apparatus needed for manipulating a tape lead to high maintenance costs.
Accordingly, attention has been directed to semiconductor integrated circuit memories, particularly MOS (for metal-oxide-semiconductor) memories. Such memories, which comprise an array of MOS flip-flop multivibrators, have become popular because of their relative ease of manufacture and consequent low cost. One drawback of conventional MOS memories is that each cell draws a significant amount of current in its quiescent condition. This can be particularly troublesome in repertory dialers because it is desirable to operate the entire substation apparatus from the central office d-c battery voltage available from the telephone subscriber lines.
Accordingly, investigation has been made of CMOS (for complementary-MOS) memories which have the known advantage requiring less power to maintain the stored data. Each memory cell, or multivibrator component, comprises both n-channel and p-channel MOS transistors, known as complementary transistors. As is known, with this arrangement, one reverse-biased transistor is included during all quiescent periods in each device current path, which considerably reduces the current flow needed for memory retention; typically the quiescent current is on the order of nanoamperes which compares with the microampere current required by conventional MOS multivibrators.
Telephone subscriber lines are unavoidably susceptible to the risk of power outage. Ordinarily, a temporary outage does not have any serious consequences, but if repertory dialers using semiconductor memories are included at the substations, the outage may result in the loss of data stored in the semiconductor memories.
Accordingly, it is known to use a capacitor in parallel with a memory for discharging, in the event of a power outage, a temporary current through the memory to maintain the stored data. While the low power requirements of CMOS memories have been made this temporary maintenance technique feasible, the maximum time such data can be retained has been about 0.8 days. Because serious disruptions sometimes cause power outages that last more than one day, it would be desirable to increase the time during which such data can be temporarily retained.